Aviation utilization of geostationary satellites for the augmentation to GPS: Ranging and data link.

摘要

The Wide Area Augmentation System (WAAS) is a GPS-based navigation aid currently under development by the Federal Aviation Administration (FAA). WAAS will provide corrections to aviation users for the GPS clock, its ephemeris, and for the delay in its signal as it passes through the ionosphere. These corrections will be broadcast to users throughout the United States via geostationary satellites. A master station that combines data from a continental network of reference GPS receivers will create these messages. The geostationary satellites serve both as wide-area differential GPS data links as well as additional ranging sources. The data message stream of WAAS enhances the accuracy and integrity of the GPS signal for aviation. Simultaneously, the satellite ranging-source increases the percentage of time that the precise signal is available. In this way, WAAS provides needed improvements in four metrics over the standard GPS signal: accuracy, integrity, availability, and continuity.; The ranging function, described above, requires an estimate of the position of the geostationary satellite. This dissertation presents a novel technique for generating this position estimate. This technique is designed to provide high integrity performance in the user position domain and operates in real-time. As such, it contrasts classical orbit determination techniques that have no integrity requirement, are not designed to optimize performance in the user position domain, and usually have no real-time requirement. Our estimator is evaluated using real data from the FAA's National Satellite Test Bed (NSTB).; The WAAS Signal-In-Space (SIS) has a limited data message bandwidth of 250 bits-per-second. This data bandwidth was chosen to balance two concerns. First, the power of the signal must not be so strong that it jams GPS. Second, the signal must provide the minimum amount of information necessary to ensure adequate accuracy and integrity for aviation users over the entire poststationary satellite footprint. The required message loss rate is specified not to exceed a rate of 0.001 (one loss per one-thousand messages) to ensure adequate system continuity and availability. The WAAS message structure is not particularly sensitive to independent message losses below the specified rate. Groups of missed messages (burst-mode) can prove to be a challenge in maintaining a continuous WAAS solution. The effects of burst-mode losses on the quality of the WAAS solution is presented and a Markov model for the burst message loss is developed. This research shows that these burst message losses can be tolerated for WAAS availability provided that the message loss rate does not exceed a rate of 0.005.; Flight tests were conducted in California and Alaska to establish actual message loss profiles for aircraft. These flight test results were modeled and used in conjunction with NSTB reference station data to establish availability of WAAS solutions for various locations in the US.